Children with chronic cholestasis have significant osteopenia. The present proposal is evaluating the role of magnesium deficiency and the effect of repletion on bone accretion in children with chronic cholestasis. Studies at baseline and after repletion with oral magnesium are evaluating magnesium status using magnesium loading tests, PTH responsiveness to hypocalcemia, 1,25(OH)2 vitamin D secretion with PTH infusion, and bone accretion rates.